Papers by Author: Hitoshi Ohsato

Abstract: Origins of high Q are considered on intrinsic as high symmetry, ordering structure and high density crystal structure. It was concluded that the high symmetry brings high Q instead of ordering comparing some cases as follows: As if ordering ratio of Ba(Zn1/3Ta2/3)O3 (BZT) is high of about 80%, Q values are distributed from low to high Q. Disordered BZT ceramics with high density obtained for short sintering time by spark plasma sintering (SPS) showed high Q. Ba(Zn1/3Nb2/3)O3 (BZN) with order-disorder transition showed high Q at disorder form sintered over the transition temperature. And, the disordered BZN with high Q annealed at lower temperature changed to order structure without improvement of Q.

Abstract: Structural characterization of various (Li,Na,K)NbO3 solid solutions with different Li contents has been carried out by temperature-variable Raman scattering measurement. It was found that there was no clear thermal hysteresis between orthorhombic-tetragonal phase transition in the heating/cooling processes for (Li,Na,K)NbO3 ceramics with Li content of 6 wt% and above, although a noticeable thermal hysteresis is observed in the Raman spectra and dielectric constant measurement for the other specimens with lower Li contents. It is presumed that the structural change of (Li,Na,K)NbO3 cell tends to become insensibility against temperature as the Li content increases above 6.0 mol%.

Abstract: The large negative temperature coefficient of resonant frequency ( τ f ) of Al2O3 is a problem for applicable microwave/millimeter wave dielectrics. A Previous study reported that the τ f was improved by sintering with TiO2 and then annealing, where the annealing decompose the secondary phase of Al2TiO5. This study investigated the hold time of annealing. The Quality factor (Qf ) value decreased when the sample was annealed at 1100°C for 2hrs. While Al2TiO5 was not detected by X-ray powder diffraction, it was located around TiO2 by scanning transmission electron microscopy equipped with energy dispersive X-ray spectroscopy. It should be considered that the degradation of the Qf was caused by the existence of noncrystalline Al2TiO5 and new boundaries between noncrystalline Al2TiO5 and TiO2.

Abstract: Mn-doped Na0.5K0.5NbO3 (NKN) crystals have been grown by self-flux method under several heat-treatment conditions. The cooling rate affected the quality of Mn-doped NKN crystals significantly. When the cooling rate was 0.5 oC/min at temperatures ranging from 1050 to 950 oC, the synthesized Mn-doped NKN crystal exhibited a single nucleation growth, compared with multinucleation growth when the cooling rate was 0.25 oC/min at temperatures ranging from 1050 to 950 oC. The frequency dependence on the ferroelectric P-E hysteresis loop of the Mn-doped NKN crystal was not observed at measurement frequency of 0.1 – 25 Hz. The annealed 0.5 mol% Mn-doped NKN crystal exhibited excellent P-E hysteresis loop with Pr of 45.0 μC/cm2 and Ec of 7.2 kV/cm.

Abstract: (Na0.5K0.5)NbO3-LiNbO3 (LNKN) is a lead-free ceramics, reported to show the maximum piezoelectric property when the Li content is 6.0 mol%. The temperature dependence of the spectra was measured for LNKN in the 4.0-8.0 mol% range of Li content by using Raman scattering. The stretching and bending mode of NbO6 octahedron especially changed the form and/or frequency of the spectrum with increasing Li content from 5.0 to 6.0 mol%. The range of this composition is corresponding to the composition with the maximum piezoelectric property.

Abstract: KNbO3 (KN) powder was synthesized by modified Pechini method (polymeric precursor method). KN powder with different crystal structure and grain size distribution was synthesized by different drying temperature. In this paper, the mechanism of preparation of KN powder was discussed in thermal decomposition behavior. The obtained powder was characterized by XRD, TEM, FT-IR and TG-DTA.

Abstract: Various KNbO3 powders have been derived from sol-gel process by using different preparation and heating conditions. KNbO3 powder derived from ethanol solution and fabricated at 800oC was most suitable as the raw material of KNbO3 bulk ceramics, because of the small grain size of about 250 nm and its homogeneous size distribution.

Abstract: The development of lead-free piezoelectric materials has been required from the viewpoint of environmental preservation. We focused the lead-free KNbO3-based systems, and used a chemical solution deposition (CSD) process to obtain their precursor. (Na1-xKx)NbO3 precursor solutions were prepared from metal alkoxides and solvents such as absolute ethanol and 2-methoxyethanol. Crystallization behavior of the precursor gels obtained from the above solutions was investigated by the thermogravimetry differential thermal analysis (TG-DTA) measurement.

Abstract: A metallodielectric metamaterial have been investigated by using FDTD (Finite Difference Time Domain) method and fabricated with a resin based rapid prototyping machine. It was composed of 7 layers of parallel periodic copper wires embedded in resin. The metallodielectric metamaterial shows a different near field distribution with direction of incident electric field E that causes different electromagnetic (EM) properties. In particular, when incident electric field E is vertical to the wires inside resin, we observe enhacement of electric field in the vicinity of the embedded metal wires according to the incident direction of electirc field E as compared with dielectirc wihout metal wires. The enhanced electric field by the embedded metal wire is responsible for the enhancement of effective dielectric constant.